The invention relates to a magnetic field generating apparatus applied to nuclear magnetic resonance imaging apparatus (hereinafter referred to as "MRI").
FIG. 5 is a diagram showing the appearance of a split type magnetic field generating apparatus for MRI (hereinafter referred to simply as the "magnetic field generating apparatus" whenever applicable) disclosed in U.S. Pat. No. 5,436,607. In FIG. 2, reference numeral 100 denotes a magnetic pole unit, which is composed of a superconducting coil 101 and a magnetic pole piece 102 made of a ferromagnetic substance. Two magnetic pole units 100 are arranged in pair so as to confront each other with the axis thereof extending in a vertical direction as shown in FIG. 5. A uniform magnetic field is generated within an imaging aperture 103 that is formed in between the pair of magnetic pole units. It is in this imaging aperture 103 that a subject (patient) is placed.
In MRI, a gradient magnetic field generating unit for obtaining positional information and RF coils (radio-frequency transmission and reception coils) are disposed between the pair of magnetic pole units, so that information about a distribution of predetermined nuclei on a desired slice of the subject can be obtained to thereby produce a tomographic image. The distance between the pair of magnetic pole units is determined by the respective sizes of a required central aperture in which the subject is placed, the RF coils, and the gradient magnetic field generating unit (an exemplary gradient magnetic field unit is disclosed in Unexamined Japanese Patent Publication No. Hei. 6-14900).
FIG. 6 is a sectional view showing a main portion of another conventional magnetic field generating apparatus. This apparatus employs no magnetic poles made of a ferromagnetic substance in order to reduce the weight of the apparatus. In FIG. 6, reference numeral 110 denotes a magnetic pole unit, which is constructed as follows. Reference numeral 111 denotes a low-temperature container which is circularly annular about an axis Z and is rectangular in cross section; 112, a collective coil body that has a side coil 112a and second to fifth coils 112b to 112e. The side coil is rectangular in cross section and is circularly annular about the axis Z. Reference numeral 114 denotes a disclike gradient magnetic field generating unit; 115, a flat radio-frequency transmission coil (hereinafter referred to simply as "the transmission coil" whenever applicable); and 116, a flat radio-frequency reception coil (hereinafter referred to simply as "the reception coil" whenever applicable).
The thus constructed magnetic pole units 110 are arranged so as to confront each other in the Z axis direction in pair. The two magnetic pole units 110 are arranged in such a manner that a distance J is interposed between the mutually confronting reception coils 116 thereof to thereby ensure a central aperture for placing a subject. The distance between the mutually confronting side coils 112a under this condition is represented by H in FIG. 6. A uniform magnetic field is generated within an imaging aperture formed between the pair of magnetic pole units, and it is in this imaging aperture that the subject (patient) is placed.
In order to give a sense of openness to the subject, it is desired that the central aperture for placing the subject be large, which in turn requires a large distance between the pair of magnetic pole units. In addition, in order to improve the tomographic resolution, a large magnetic field output is required. It is for this reason that the size of the magnetic pole unit 10 tends to increase.
When the distance between the pair of magnetic pole units is increased, a magnetomotive force required by the magnetic pole 10 is also increased. FIG. 7 is a graph showing a relationship between the inter-coil distance H (H/2) and magnetomotive force in a magnetic field generating apparatus whose central magnetic field is 0.5 tesla. In FIG. 7, reference character H denotes the inter-coil distance including the imaging aperture. The abscissa is graduated with H/2 as a reference. It may be noted that FIG. 7 indicates both a nonshield type (denoted as N/S) and an active shield type (denoted as A/S). The nonshield type has no shield coils, so that the magnetic field of the magnetic field generating apparatus leaks outside. The active shield type has shield coils that control leakage of the magnetic field to the outside.
It is understood from FIG. 7 that a required magnetomotive force is proportional to substantially the 5th power of the inter-coil distance, whether the magnetic field generating apparatus be of the nonshield type or of the active shield type.
As described above, when the distance between the mutually confronting magnetic pole units, i.e., the distance between the mutually confronting collective coil bodies is increased in the conventional split type magnetic field generating apparatuses, a required magnetomotive force is increased to a significant degree, which in turn has imposed the problem of increasing the structure of the coils and hence the price and weight of the apparatus.